(1) Field of Invention
The present invention relates generally to devices for the accurate and repeated automatic or robotic control of the movement and positioning of a toolhead relative to the surface of a workpiece for carrying out computer-controlled programmed instructions for manufacturing operations.
(2) Description of Prior Art
The acronyms CNC and CAM stand respectively for Computer(ized) Numerical(ly) Control(led) and Computer Aided Manufacturing, and refers specifically to the computer control of machine tools for the purpose of (repeatedly) manufacturing complex parts in metal as well as other materials.
The introduction of CNC machines radically changed the manufacturing industry. Curves were as easy to cut as straight lines, complex 3-D structures were relatively easy to produce, and the number of machining steps that required human action diminished drastically.
In a production environment, all of these machines may be combined into one station to allow the continuous creation of a part involving several operations. CNC machines are driven directly from files created by CAD (Computer Aided design)/CAM software packages, so that an assembly or part can go from design to production without any intermediate paper drawing work being required. In one sense, CNC machines may be said to represent special industrial robot systems, as they are programmable to perform any kind of machining operation (within certain physical limits, like other robotic systems). CNC machines were relatively briefly preceded by the less advanced NC, or Numerical(ly) Control(led), machines.
A computer numerically-controlled (CNC) machine tool utilizes computer controlled motors in addition to the position feedback signals to precisely machine components. They may machine components using simultaneous multi-axis coordinated motion. Once the part program (computer software) is prepared, CNC machines can run unattended.
The machining objective loaded into the controller identifies the current operation as a positioning, contouring or pocketing operation. The machining constraints associated with the current objective are also loaded into the controller. These constraints may differ for each objective, as will be explained shortly. Position transducers (sensors) mounted on the machine tool issue position feedback signals which indicate their positions.
For contouring operations, a path constraint and a tolerance are specified by the machinist. This path may be in the form of a line/arc definition of the desired trajectory, a mathematical function or any other format that lends itself to manipulation by a computer. The tolerance value, identifying the locus of points that are within a known distance from the path movement, is used to establish the tolerance zone in which the tool motion is permitted. This information along with the position feedback signals is used, in part, to issue control signals to the motion switch means.
When a positioning operation of the tool is desired, the machinist enters a position constraint and tolerance constraint for each controlled axis collectively defining a target region. These constraints are used to issue control signals to the motion switch means based on the position feedback signals for each axis as transmitted by the position transducers.
Pocketing operations require the machinist to define boundary surface constraints and may include a tolerance or roughing distance. The “boundary surface constraints” define surfaces with which the tool is not permitted to come into contact and the “roughing distance” defines how close to the boundary surface constraints the tool is permitted to be moved. These constraints and the position feedback signals are used, in part, to issue control signals to the motion switch means, enabling and disabling the relative motion of the tool.
Computer-controlled carving machines, referred to as “CNC routers,” have been commercially available for some time. CNC routers are expensive and large relative to the size of the workpiece that they can be employed to shape and rout.
CNC routers suffer from a number of deficiencies, in addition to large physical size relative to the maximally sized workpiece on which they can operate. First, the large bed required to support large workpieces adds considerably to the cost of CNC routers. The large bed size also adds considerable weight to the overall weight of CNC routers, since the large bed must be thickly cast or otherwise rigidly constructed to avoid sagging and other shape alterations. CNC routers require stiff and rigid components, because positional accuracy of the cutting head under computer control is possible only when x, y, and z translations of the cutting head predictably and reliably position the cutting head with respect to the bed, and the workpiece affixed to the bed. In general, CNC routers employ non-intuitive, and difficult-to-learn operator interfaces, and programming of CNC routers generally requires considerable training.
CNC routers, despite their disadvantages, have enormous usefulness in woodworking and in carving and shaping other rigid and semi-rigid materials. Wood workers, manufacturers, carpenters, artists, hobbyists, and others who carve and shape rigid and semi-rigid materials have thus recognized a need for a cheaper, smaller, lighter, and easier-to-use processor-controlled carving and shaping device.
A conventional vertical milling machine is equipped with a horizontal table for holding a workpiece, and a power-rotated cutter for machining the workpiece. The table and the cutting tool of a typical three-axis machine are adapted for relative longitudinal movement along a horizontal X-axis, relative lateral movement along a horizontal Y-axis, and relative vertical movement along a Z-axis. The cutting tool in such machines is typically positioned in a spindle in a vertical position, and in certain machines in a horizontal position or in an adapter for selecting either the vertical or horizontal position. In certain high-performance 4-axis machines, the cutting tool is located in a motorized spindle adapted for pivoting about a horizontal axis A, and in 5-axis machines, for rotation about a vertical axis C.
A conventional vertical turret lathe is equipped with a horizontal table mounted for continuous rotation of the workpiece about a vertical axis, and a non-rotating cutting tool typically positioned in a horizontal position. The table and cutting tool are typically adapted for relative positioning along the X and Z-axes for positioning of the workpiece on the table and machining of the rotating workpiece.
Numerous variations of milling machines and turret lathes are known in the art, as well as several machines that have attempted to merge the benefits of these two types of machines. One prior type of machine includes an adapter permitting removal of the turret lathe horizontal tool holder and installation of a milling vertical tool holding spindle to effect conversion from turret lathe operation to milling operation. Another prior type of machine provides for a turret lathe and a milling station in close proximity in the same machine to reduce transfer time between the two stations. Yet another type of vertical milling machine has been provided with a table mounted for rotation about a vertical axis and for swiveling about a horizontal axis.
However, these as well as other prior machines have failed to achieve an effective combination of the machining capabilities of milling machines and turret lathes, coupled with the necessary quick response times, such that the resultant machine is suitable for precision, high-speed CNC milling operations as well as general turning purposes of a conventional turret lathe.
It is conventional in the U.S. automotive industry to shape a complex workpiece, such as an engine block or head, by transferring such workpiece, clamped on a fixture and pallet, along a series of machining stations where a specific surface is cut or finished by a dedicated tool (or cluster of dedicated tools) fed along a unitary axis. The workpiece must be transferred, with time-consuming effort, to other fixtures and/or pallets to expose a variety of faces to the feed axis of the tools. The percentage of in-cut time exercised by such a system is low due to the frequency of low speed workpiece transfer and due to the slow rates of toolhead positioning. Each toolhead carries out a task dedicated solely to one machining function with little modification over several years of use. The initial cost of fabricating and installing such nonflexible dedicated equipment with complex controls is very high not only due to their sophistication but also due to the large number of single purpose cells needed to complete the shaping of a specific engine block or head.
While machining devices may be manually or computer-controlled, CNC milling machines are increasingly performing the greatest proportion of such milling tasks. Typical CNC milling machines have a machine spindle head with a rotating spindle shaft that handles a plurality of machining tools, including drills and many styles of chip removing cutters. When these CNC milling machines include a mechanism for exchanging of these chip-cutting tools, they are generally referred to as machining centers. These milling machines and machining centers are designed to produce a finished workpiece from the raw starting material as quickly and precisely as possible. Machines have been developed to operate as fast as possible, and milling tools are designed to efficiently remove large quantities of waste material through their cutting actions. When an exchange of tools is required, the interruption of the machining operation for the tool exchange function is typically so short that little time is added to that of actual machining.
The spindles on the most common CNC milling machines have either a vertical or horizontal orientation that sets the manner in which the milling cutters address the workpiece. It is obvious that workpieces may require milling from more than one side, and such workpieces require extra operations that may include repositioning of the workpiece on the machine's workpiece mount so that the cutters can address the other sides of the workpiece. This repositioning of the workpiece causes a loss of time and accuracy in the operations. Some milling machines have more than one tool-driving spindle, with secondary spindles being able to work on the same or other sides of the workpiece. Milling machines that have these secondary spindles are, however, of special and expensive construction, and as such are less common in the industry.
The most complex and expensive milling machines respond to these problems by the inclusion of mechanisms which tilt the spindle or the workpiece about one more axes, thus allowing the cutters to address the workpiece from more than one side. These complex machines, often called universal or 5-axis milling machines, while versatile in achieving many angles of milling, lose rigidity in the tilting mechanisms, accommodate relatively smaller workpieces, and must be constructed with great and costly care to achieve accuracy.
U.S. Pat. No. 4,146,966 to Levine et al., Apr. 3, 1979, discloses an engraving machine for rings with a rotating workpiece. The limited range of movement does not provide for movement in perpendicular rotational axes around the workpiece.
U.S. Pat. No. 4,848,942 to Speicher, Jul. 18, 1989, discloses a device utilizing impact pins to mark an arcuate surface. The limited range of movement does not provide for movement in perpendicular rotational axes around the workpiece.
U.S. Pat. No. 5,190,384 to Speicher, Mar. 2, 1993, discloses a dome and round parts rotary marker utilizing impact pins to mark an arcuate surface. The limited range of movement does not provide for movement in perpendicular rotational axes around the workpiece.
U.S. Pat. No. 5,203,088 to Morgan, Apr. 20, 1993, discloses a device for engraving an article with a curved surface (e.g., rings). The limited range of movement does not provide for movement in perpendicular rotational axes around the workpiece.
U.S. Pat. No. 6,145,178 to Green, Nov. 14, 2000, discloses a milling machine with horizontal and vertical spindles. The limited range of movement does not provide for movement in the rotational axes around the workpiece.
U.S. Pat. No. 6,502,002 B2 to Susnjara et al., Dec. 31, 2002, discloses a multi-purpose, flexible CNC machine for performing a variety of machining operations on all parts of a product. It does not provide for six degrees of freedom of movement utilizing only five axes of movement.